Fourth-order reaction

At lower concentrations of base it is a mixture of both third- and fourth-order reactions, rate =k3[HCH0]2[H0 ] + k4[HCH0]2[HO ]2... 

At concentrations below 0,4 vaM, a fourth-order reaction at the crystal surface controls ... 

Kinetic studies of the acylation of imidazoles with benzoyl halides and sulfonyl halides have been carried out. In acetonitrile, benzoyl fluoride reacts with imidazole in a mixed third- and fourth-order reaction i.e., the rate-determining transition state contains the acid fluoride and two or three molecules of imidazole. With benzoyl chloride in benzene a second-order equation is followed similar to the aroylation of anilines ... 

The rate of reduction reaction is a fourth-order reaction with respect to electron concentration ... 

The experimentally determined reaction rate equation is fourth-order and dependent on [Fe " ], [O2], and [OH ] (Table 9.3b). The rate dependence on [Fe and [O2] was determined by measuring the rate of disappearance of Fe(II) (Roman numerals are used to indicate the oxidation state without specifying the actual species in solution) at various pHs while O2 remained constant (and vice versa for the O2 dependency) (Stumm and Lee, 1961). We loiow from previous discussion that the fourth-order reaction mechanism in Table 9.3b would be nearly impossible. Millero et al. (1987a) suggested a rate-limiting mechanism that is a second-order reaction between... 

Overall a carboxylate anion and an alcohol have been formed from two molecules of aldehydes, and so a disproportionation reaction has been effected. There is an alternative mechanism in which the rate is found to depend upon the square of the concentration of the base and the square of the concentration of the aldehyde, i.e. it is a fourth order reaction. In this pathway, the base removes both the hydroxyl protons to form the dianion, which then donates a hydride ion to the other aldehyde. 

Kinetic studies of the acylation of imidazoles with benzoyl halides and sulfonyl halides have been carried out. In acetonitrile, benzoyl fluoride reacts with imidazole in a mixed third- and fourth-order reaction ... 

The rate is actually given by /[S20g ][I ] (a second-order reaction) and not k/[S20g ] [ 1 ], as might be expected from the balanced chemical reaction (a fourth-order reaction would be predicted). The only sound theoretical basis for the equilibrium constant comes from thermodynamic arguments. See Gibbs free energy in Section 6.3 for the thermodynamic computation of.equilibrium constant values. 

We and others have demonstrated that association of short strands containing a single guanine-repeat seems to obey a fourth-order kinetics model. Third or fourth-order reactions are not common in biochemistry, and the practical consequences of this reaction order are important. A fourth-order reaction does not imply that an elementary kinetic step involves a four-body collision. Such mechanism is extremely unlikely and other processes could lead to this fourth order. The structure of these elusive intermediates remains unknown Stefl et have recently demonstrated that a Hoogsteen G-G duplex is an improbable intermediate. Its identification will be experimentally difficult, as numerical simulations indicate that it may not be present at detectable levels. 

Yen eta/ [1988] Fluid catalytic cracking Coke formation is kinetics control with fourth order reaction None considered (1) Complex - many parameters (2) Tested against actual coke data from both pilot and commercial fluid catalytic cracking units... 

Most reactions are bimolecular and only a few are either monomolecular or ter-molecular. When the overall reaction order is greater than three, it is likely that it is a chain reaction with one or more equilibrated intermediates prior to the rate-determining step. For the reactions 2A + 2B = 2AB (fast) and 2AB —> 2C (slow), the empirical rate equation might be written as r = km ml and appear to be an overall fourth-order reaction but it would be more meaningful to write the rate law in terms of the intermediate AB, i.e. r = km g. 

Give some examples of Third Order Reactions and one example of a Fourth Order Reaction. 

---